This invention relates to tracking systems for aiming a laser beam and/or positioning projected light patterns at a known relation onto moving targets. In particular, the invention is concerned with the detection of, measuring of and compensating transverse movements of optical targets such as an eye during ophthalmic laser surgery as well as non-surgical diagnostic procedures. The present invention is particularly powerful when taken in conjunction with a method for capturing, measuring and compensating for movements along the axial direction such as disclosed by Wm. D. Fountain in U.S. Pat. No. 5,162,642 which is assigned to the same party as the present invention.
Methods of integration of a lateral tracker with depth ranging techniques were disclosed in copending patent application Ser. No. 843,374, entitled "Automated Laser Workstation for High Precision Surgical and Industrial Interventions", which was filed on Feb. 27, 1992, and which is incorporated herein by reference. In that disclosure, fully automated means to acquire and track randomly moving targets in three dimensions were described, along with methods for interfacing the acquisition and tracking means with a beam aiming and targeting sub-system and a target viewing subsystem, all of which are elements of a complete laser workstation. It is noteworthy that the system and method of said patent application also included, in an alternate embodiment, the capability to distinguish between translational and rotational movements of the eye as an integral part of the full three dimensional tracker.
By comparison with the two earlier disclosures cited above, the present invention emphasizes those aspects and specific embodiments of a transverse 2D tracker that are most critical in allowing a laser beam or projected light patterns to be correlated with and/or directed to a specific location on the target regardless of its lateral movement. Since the application to eye surgery places the most stringent requirements on the tracker, the present invention is described mostly in reference to this application. However, it is to be understood that the invention is broadly applicable to any situation involving precision diagnostic measurements and/or laser operations on moving targets, including industrial applications, such as in semiconductor processing where laser annealing and other techniques call for precise alignment of a mask onto a substrate in the presence of vibrations.
In ophthalmic surgery, the ability to optically track or follow the movement of the patient's tissue--not only the voluntary movements which can be damped with specialized treatment, but also the involuntary movements which are more difficult to control on a living specimen--is recognized as a highly desirable element in laser delivery systems designed to effect precision surgery in delicate ocular tissue. According to Adler's Physiology of the Eye, even when the patient is holding "steady" fixation on a visual target, eye movement still occurs. Such involuntary motion compromises the efficacy of certain ocular surgical procedures requiring great precision. This is true even with total immobilization of the eye, which is not fully effective in suppressing involuntary eye motion while being rather uncomfortable for the patient. Implementation of automatic tracking by remote means would therefore alleviate the need for such immobilization, while offering a method for more effectively accomodating all types of eye motion. Thus, augumenting surgery with on-line eye tracking option can improve significantly upon the accuracy and speed with which old surgical procedures could now be performed as well as enabling new procedures to be carried out for the first time.
In ophthalmology, it is also often desirable to image the tissue simultaneousely with positioning the treatment beam. Effective utilization of an imaging system capable of freezing on a display images or data relating to the configuration of the target during laser treatment requires that the target area be stabilized with respect to both imaging and the laser focal region, thus enhancing the accuracy of energy deposition in tandem with viewing sharpness. The ability to stabilize a video image of a moving target during the surgery procedure itself is especially desirable in those high precision laser interventions employing an instantaneous full image rather than a series of scanned images, such as described in co-pending U.S. patent application Ser. No. 843,374, which is incorporated herein by reference, and in U.S. Pat. No. 5,098,426.
In still other applications relating to diagnostics of targets, tracking can serve an important function in allowing cross-registration of successive readings taken across a moving target. By correlating the true positions of given target segments at the time the readings are taken, the effects of target motion can be compensated for via programming in the computer (i.e., software). In application to corneal surface mapping, utilization of transverse tracking, especially in concert with a depth tracking method that can keep the distance to the eye constant, e.g., as was disclosed in co-pending U.S. patent application Ser. No. 945,207, opens up the prospect of performing true point-by-point thickness and curvature measurements with standard scanning techniques. For example, by aligning separate readings relative to each other, accurate reconstruction of both anterior and posterior surfaces of ocular tissue such as the cornea or the lens can be feasible by scanning the eye with just a slit illuminator coupled to a CCD camera to detect each surface's reflections. Using such simple instrumentation to perform simultaneous surface and thickness measurements was not possible prior to this invention.
Prior attempts to derive simultaneous pachymetry and topography information such as by D. J. Gormley et. al. in Cornea, vol. 7, pp. 30-35, 1988 using a scanning laser slit lamp and a photokeratoscope were clearly hindered, among other factors, by the lack of cross-referencing that only tracking can provide. Thus, to compensate for eye movement, each slit lamp image reading consisted of an average of a series of measurements, a procedure which could take up to several minutes. To map the entire cornea in this manner would then clearly require an inordinately long time, especially since, to maintain a common reference point between successive readings, the patient would have to stay fixated throughout the entire scan. By adding means for on-line tracking, the number of required readings can be significantly reduced to where a combined method of depth and surface profiling becomes practical, even allowing the possibility of utilizing a slit illuminator alone for both measurements.
In general, stabilization of a moving target requires defining the target, characterizing the motion of the target, and readjusting the aim of the optical system repeatedly in a closed-loop fashion. For ophthalmic surgery, requirements for a tracking system are set by the type of eye motion, which fall into three categories: microsaccades, drift and high frequency tremor. The high frequency tremors, of about 90 Hz set an upper limit to the frequency, but are of a very small amplitude (up to 40 seconds of arc). Microsaccades are highly accelerated motions with constantly changing directions but lower frequencies (a few Hz). These small but rapid eye movements, combined with slow drift (about 1 minute arc/sec), prevent the retinal image from fading. Analysis of measurements of peak velocity-magnitude-duration parameters by e.g., A. Terry Bahill et al. in Invest. Ophthalmol. Vis. Sci., Vol. 21, pp. 116-125, 1981, indicate that requirements set for lateral eye tracking should include, as a goal, the ability to respond to movement with accelerations of up to 40,000 deg/sec.sup.2. This translates to amplitudes of about 1 degree at maximum frequencies of 100 Hz and increasing to nearly 15 degree at 20 Hz. Meeting these response goals while maintaining accuracies on the order of 5-10 microns (as may well be desired in certain ocular surgical interventions near the visual zone), means that any moving parts within the tracker apparatus must not contribute internal vibrations, overshoots, or other sources of positioning error which could cumulate to an error in excess of the prescribed micropositioning tolerances. Other applications may impose tighter requirements on some parameters while relaxing others, depending on the margins of error set for a given process or procedure and the required overall system performance. We do expect, for example, that compensation over larger amplitudes and/or at higher frequencies may be desired in certain micromachining applications, which tighter requirements may, however, be offset by the ability to mark the workpiece to produce sharper tracking landmarks. Such target signal enhancements are not possible for ocular tissue without invading the organ itself, which is what makes the eye tracking application so uniquely demanding.
To date, no instrument has succeeded in tracking the human cornea in a cost effective manner. Previous attempts at achieving this result fall into one of two distinct categories, namely optical point trackers and correlation trackers, the latter including numerous variations of pattern recognition and edge detection methods. Optical point trackers utilize various lens-like properties of the eye to locate optically distinguished locations such as the first, second, third and fourth Purkinje points. For example, Crane and Steele (Applied Optics, Vol. 24, pp. 527, 1985) describe a dual Purkinje projection technique to compare the displacement of two different-order Purkinje projections over time, and a repositioning apparatus to adjust the isometric transformation corresponding to the motion. Similar application of dual Purkinje technique to a stabilized visual system was advanced by Crane (U.S. Pat. No. 4,443,075) using a fundus illumination and monitoring device. These and similar Purkinje image-based tracking methods purport to follow the movement of the anterior surface of the eye. While such techniques possess, in principle, sufficient speed to follow the displacement of Purkinje points, they do include an implicit assumption that the eye moves as a rigid body. In reality, however, the eye does not move as a rigid body, hence localization of the Purkinje points can be influenced by transient relative motions between the various optical elements of the eye, which leads to fictitious position information for identifying the surface of the cornea. In addition, such systems are rather complex and tend to exhibit large interperson variability in their calibration setting, which requires continuous real-time adjustments of the amplitude of the controlling signals.
The other class of tracking methods suggested in prior art involve, in one form or another, digital correlation techniques. These include retinal image trackers and various pattern recognition algorithms involving edge detection techniques. In either case, very fast frame-rate CCD cameras and sophisticated processing algorithms are required along with fast servo-controlled mirrors closing the loop. This is because, in general, methods based on pattern recognition are fundamentally digital, requiring high frequency updates. With the frequency response limited in practice to about one tenth the update frequency, digital signal comparisons are considered to be relatively slow. In the case of tracking eye motions, setting the sampling frequency to about an order of magnitude higher than the highest frequency to be pursued translates into kHz rates, leaving less than one thousandth of a second for processing the signal information.
Several other practical difficulties plague most pattern recognition techniques including the need for rather prominent and recognizable features, which are not easily located in any of the eye's structures. Also, techniques predicated upon high speed correlation processing of video signals are often deficient due to unfavorable trade-offs between field of view and spatial resolution. Specifically, since the image processing algorithms are limited by the size and spacing of the view elements (pixels), the digital methods do not afford continuous resolution. Increasing the resolution exacts penalties in terms of the field of view. Yet, relatively large areas must be acquired or else a beam must be scanned. Consequently, the system is either light starved in the former case or else requires extremely regular and fast moving optical deflectors in the latter, along with complex electronic processing which can further limit the already slow response time of the system.
While there have been some claims of successful tracking for the retina (see for example, Milbocker and Feke in Applied Optics. Vol. 31, pp. 3719-3729, 1992, Katz et al. in American Journal of Ophthalmology, vol. 107, pp. 356-360, 1989 and A. Forster in Proc. of Ophthalm. Tech., SPIE vol. 1423, pp. 103-104, 1991), to our knowledge no instrument of this type has produced a real-time stabilized image of the cornea to date. Typically, CCD cameras are used to analyze the translations of an image on the retina from which the resulting coordinate transformation could be computed. The data is then fed to e.g., galvanometric driven mirrors which are repositioned so as to maintain a beam at the selected location of the subject. Other than the issues alluded to above regarding inherent limitations of digital edge detection techniques, the need in most of these instruments to utilize galvanometer drives to reposition mirrors at nominal kHz acquisition rates adds complexity to the system while further limiting the positioning accuracy due to overshoot problems.
Bille et al. (U.S. Pat. No. 4,848,340) describe a simpler variant on the pattern recognition method, whereby a grid is marked, using a laser, on the epithelial surface of the cornea, in a known reference alignment to the eye's visual axis. An infrared optical system monitors reflections therefrom, generating an error signal whenever the position of the mark deviates from reference alignment. The error signal is transmitted to a laser guidance system containing a fine tuner (consisting, e.g., of one or more galvanometrically controlled mirrors) which steers the laser beam in a manner that reduces the error signal to null. This type of a tracking system requires a sensor such as a photodiode array that can detect variations in the intensity of the reflected light pattern to generate signals representative of grid movements. These sensors suffer from both slow response times and limited spatial resolutions. Typically, with accuracy bounds set by the space between the array elements divided by the magnification, it is difficult to obtain resolutions better than 50 microns or so in practical systems. Furthermore, like any closed-loop feedback control that requires comparison of input signals to some reference, the technique taught by Bille et al. is digital in nature, which means that it suffers from similar drawbacks as image trackers and edge detectors in general, including processing speeds limited by the servo rate to less than a millisecond.
It should also be appreciated that a target tracking and laser positioning mechanism that relies on a mark on the surface of the cornea in order to perform corneal surgery, such as described by Bille's tracking method, might lead to misdirected positioning of laser lesions below the surface when combined with any suitable focused laser. Thus, the mark would change its absolute location due to changes in the structure and shape of the material being modified that are caused by use of a laser surgery instrument itself, rather than by eye motions. It is therefore not clear that a tracking method based on marking the target tissue itself is compatible with laser surgical interventions performed simultaneousely with the tracking. Moreover, one of the features of the present invention is to enable non-incisive procedures inside target tissues by remote means. It would hence be counterproductive to mark the surface of the cornea for the sole purpose of following the motion of said mark.
In another embodiment of Bille et al. U.S. Pat. No. 4,848,340, the tracking is based on a reference provided by an empirically determined offset between the eye's symmetry axis and the eye's visual axis. It is claimed that tissue can be tracked by monitoring the reflection from the apex of the cornea, thus avoiding the need to mark the eye, and/or, rely solely on patient fixation. However, with this technique, the tracking does not follow tissue features generally corresponding to the targeted surgical site itself. Instead, Bille et al.'s technique tracks reference points that are, like Purkinje points, a property of the optical system and do not correspond to any particular physiological tissue. They are therefore separate, remote from and may be unrelated to the targeted surgical site. The accuracy of the tracking is thus compromised in direct proportion to the degree of the reference points' remoteness relative to the surgical site, while ambiguities inherent in measurements of the symmetry and/or the visual axis will further reduce the accuracy with which positional changes of the targeted surgical site can be pursued using these methods.
By contrast with either pattern recognition based systems or optical point trackers, the methods of the present invention disclosed herein involve contrast tracking which does not rely on well-defined edges and/or patterns that must be compared to some reference. This allows great flexibility in selecting the tracking landmark, since prominent and constant edges are not required for acceptable signal-to-noise ratios. In the preferred embodiment, the tracking information is to be obtained through means contiguous to the target region, which is mechanically and structurally considered as part of it, but is unlikely to be affected by the course of the laser intervention. For example, the system and techniques disclosed herein resolve, for the first time, difficulties associated with previous attempts at limbus tracking. The limbus, located at the outer edge of the cornea, presents several advantages as a tracking landmark for corneal procedures. It is actual tissue that is contiguous to the targeted corneal tissue and is expected to provide a faithful representation of non-surgically induced displacements. Yet it is located far enough from the site of operations so that the transient displacements occasioned by the impact of the laser pulse on the target site will be damped sufficiently to avoid inducing fictitious tracking signals. Prior limbus trackers have not been successful because the limbus is a poor candidate for any technique relying on edge detection, constituting not a sharp boundary but a transition zone between the cornea and the sclera. Therefore, poorly defined edges and shapes that appear to deform due to rotations have led to difficulties in extracting signals out of noise.
The present invention overcomes these shortcomings because it is practical even in the absence of prominent, well-defined or even temporally constant edges. The only two requirements are that sufficient contrast be present, and that the feature possess a degree of symmetry. In the eye, these conditions are fulfilled by e.g., the limbus structure and in most cases, the pupil as well. In our co-pending patent application Ser. No. 843,374, a method for tracking the limbus was disclosed relying on a set of two quadrant detectors as the position sensor. The present patent application expounds on that disclosure by highlighting a unique electronic control system that can be used for the tracking to great advantage, and including as a desirable feature a dual feedback loop that can be all analog, thus significantly increasing the practical speed of operations over comparable digital methods. Along with the added emphasis on the electronic means for realizing rapid limbus tracking, the simplified signal processing and fast logic operations involved in the electronic servo loops of the present invention also allow a substantial expansion of the scope of the previously disclosed limbus tracking method to include other contrast-based tracking landmarks and alternative position sensing detectors as may be needed to implement tracking in different surgical, diagnostic or industrial settings.